<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<body>
  <button id="btn1">按钮1</button>
  <button id="btn2">按钮2</button>
  <button onclick="demo()">按钮3</button>

  <script type="text/javascript">
    const btn1 = document.getElementById('btn1')
    btn1.addEventListener('click', ()=>{
      alert("按钮1被点击了")
    })

    const btn2 = document.getElementById('btn2')
    btn2.onclick = () => {
      alert("按钮2被点击了")
    }

    function demo(){
      alert("按钮3被点击了")
    }
  </script>
</body>
</html>